The invention relates to a method for adjusting convergence in a projection television receiver and in particular to an automated convergence adjustment method.
In a projection television receiver, it is necessary to adjust the convergence during production or automatically in each case when switching on. This means that the three pictures projected onto the picture screen for the primary colours R, G, B must be brought to coincidence for each point of the picture.
It is known to image on the screen a grating or grid pattern comprising horizontal and vertical lines which forms a multiplicity of intersecting points. Convergence correction values are determined at each intersecting point of the grid. As a rule, there are six values for each point, specifically values for each colour, R, G and B, in the horizontal and vertical scanning directions. The correction values for each intersection point are stored in a digital memory. During reproduction, the correction values are extracted from the memory and converted into analogue correction values by digital/analogue converters, and used to correct convergence at each intersecting grid point. The correction of the convergence between the grid points in the horizontal and vertical directions is performed as a rule by low-pass filtering or by interpolation of the correction values.
It is also known to fit sensors in the form of photodiodes adjacent to the screen either inside or outside the visible picture, as depicted in FIG. 1B. A so-called marker block in the form of a monochrome image, that is to say red, green or blue picture point or marker is inserted into the video signal to be projected to form a picture on the screen. For optimal convergence, in each case for the primary colours R, G, B and in the horizontal and vertical directions, this projected marker on the picture screen must in each case impinge on an assigned sensor.
The adjustment of the marker to the sensor is achieved by influencing the deflection in the picture tubes which are subject in practice to a multiplicity of errors such as, in particular, drift, background brightness, sensitivity, changes in threshold value, de-focusing and lens errors. One difficulty and inaccuracy consists in the following:
When the projected marker is located beyond the sensor it does not illuminate the sensor which consequently does not output a marker generated signal. Hence the convergence correction circuit has no information as to whether the projected marker is located on the left or right of the sensor in the case of the horizontal convergence, or below or above the sensor in the case of the vertical convergence, nor as to the direction in which it must move to find the sensor. There is then a need for a search, which can, with a certain probability, lead in the wrong direction. In a multiplicity of cases, the marker therefore has to continue the search in the opposite direction if it does not find the sensor in the first adjusting direction. This means a loss of time, which can be perceived as disturbing during switching on, in particular in the case of automatic convergence adjustment.
It is the object of the invention to provide a method for adjusting the convergence in a projection television receiver, in which the said errors are corrected. In one embodiment of the invention, it is achieved, furthermore, that at the start of the convergence correction the marker automatically carries out a movement in the correct direction relative to the sensor and a search operation in the wrong direction is avoided.
The essence of the invention is thus that two markers are moved towards the sensor from opposite directions with a varying step size until the sensor supplies an output voltage indicating light for the two markers in conjunction with two mutually spaced manipulated variables, and in that a mean value of the two manipulated variables is used as manipulated variable for the marker.
Thus, in the case of the invention the marker approaches the sensor by using markers which approach the sensor from opposite directions. The finite dimensions of the marker and the sensor thus provide two mutually spaced manipulated variables because, for example, in the case of the horizontal convergence the marker impinging on the sensor from the left and the marker impinging on the sensor from the right necessarily impinge on the marker in conjunction with different manipulated variables in the horizontal direction. The optimum manipulated variable can thus be determined by averaging between these two manipulated variables, and can be used for the convergence.
In one embodiment of the invention, the area of the marker is large by comparison with the area of the sensor. The marker preferably has a rectangular or square area comprising a multiplicity of successive lines with the whole line duration or a part thereof. The dimension of the marker in the direction of the adjustment relative to the sensor is larger in this case than the adjustment range of the marker, both for the horizontal convergence and for the vertical convergence. The dimension, the adjustment range and the position of the marker relative to the sensor are matched to one another such that in its end setting in the adjusting direction towards the sensor, the marker does not leave the sensor, and in its end setting in the adjusting direction away from the sensor it leaves the sensor. The area of the marker is preferably equal to the area which is enclosed by grid lines of a grid pattern which is represented on the picture screen and defines convergence intersection points.
By virtue of this embodiment of the invention, it is possible, as will be explained in more detail in the description, to achieve that in each case a marker can already detect at the instant of the start of the convergence correction, without a movement and purely on the basis of its output signal, whether it is on the left of the sensor and thus must be moved to the right, or whether it is located on the right of the sensor and therefore must be moved to the left to find the sensor, in order to strike the sensor. As a result, time is saved in adjusting the convergence, for example in each case when switching on the set, and convenience for the customer is enhanced.
Another embodiment of the invention operates using the following steps:
a) the manipulated variable is changed in large steps such that the marker is moved from the first side towards the sensor until, in conjunction with a first value of the manipulated variable, the sensor supplies a signal triggered by the marker,
b) the manipulated variable is moved back by one step to the second value, situated therebefore,
c) the manipulated variable is changed again in the direction towards the sensor in smaller steps, until the sensor again supplies a signal in conjunction with a third value of the manipulated variable,
d) the steps a)-c) are carried out likewise by moving the marker from the other side towards the sensor, a fourth value thereby being produced, and
e) the mean value between the third value and the fourth value is used as manipulated variable for the convergence.
The step width of the large steps is in this case approximately 5-10 times the step width of the smaller steps. The marker can be formed by a monochrome, bright line in a background which is dark in the adjustment range. The large steps preferably have a step width such that the marker cannot jump over the entire sensor with one step. This embodiment of the invention permits the particularly accurate alignment of the marker onto the sensor, the influence of the inaccuracy parameters mentioned at the beginning on the convergence correction being eliminated to the greatest extent.